William Allman: There's a surviving letter from a Florentine merchant named Francesco Lapi and he wrote in 1535 that he was expecting a big shipment of wine in somewhere, Venice I think it is. And he used the symbol of the wine was the @ sign, you can see it right there on the document. So that @ sign eventually evolved to become known as "something per something," and the @ means "per" or "each" And it's been used for centuries in that basic accounting/merchant way.

Hill: And so I understand from your article there was a time when it sort of fell out of favor. What happened to the @?

Allman: It was used for a long time, and people used it in business and things. And then when the first typewriters rolled out at the beginning of the industrial revolution in the late 1800s, as you can imagine, there was scarce space on the little typewriters and people were trying to figure out what was really critical, and the @ sign wasn't even on them. You can find old typewriters still that don't have them on there. In the same way that it's hard to find a "cents" sign on a keyboard now.

Hill: So the @ sign was almost the "cents" sign?

Allman: Yes, it almost became the "cents" sign. And the person who rescued it was a fellow name Ray Tomlinson. He worked at a company called BBN and back then they were trying to solve how to get computers to talk to each other. So they hooked people up and programmers via teletype. And a teletype is basically a glorified typewriter that's connected to a phone line that talks directly to a computer. Now, what Tomlinson was doing was he said, you know, I'm connected to this computer A through this teletype machine, my friend Joe is connected to computer B through this teletype machine. And I can't get a message to him. And so he came up with a way to do a program that he could actually send a message from his teletype to a mainframe computer. That computer would then shift it over to another mainframe computer, and then it would then shift it back down to a person at the other end.

Hill: Which is basically the beginning of email, right?

Allman: It's the beginning of email. And what's critical was that Tomlinson was looking around saying, well, I have to figure out how to separate the person from the computer because there could have been multiple people on each computer.

Hill: Right.

Allman: And the network had to know it is person A at computer B going to computer C and delivered to person D.

Hill: And so how did he choose the @ symbol?

Allman: The biggest problem they had was there were lots of programming languages at the time, and most of them used a lot of the symbols for various functions. So he had to find something that wouldn't confuse the heck out of the computers. So he looked around on his keyboard and basically the only thing left was sort of the '=' sign and the @ sign, and he used the @ sign.

Hill: And so because the @ sign was so obscure, it got chosen?

Allman: Exactly. It was precisely because it wasn't used very much at all that it became this rock star of communication.

Hill: So what's the significance of the @ sign today?

Allman: It's the juncture that connects people to each other. The wonderful thing about it is he invented a way that people communicate with anyone else on the planet almost instantly and the @ sign is the lynchpin that holds people to their domain so they have their address.

And because it was obscure, because it wasn't used very much at all, suddenly got flipped. And it's one of those great 180-flips that technology does often on you where something small suddenly becomes very big precisely because it was originally small.

Hill: William Allman is the author of "The Accidental History of the @ Symbol," in this month's Smithsonian magazine. He's also the Smithsonian's chief digital officer.